The present invention relates generally to a method for preparing a .gamma.-Fe.sub.2 O.sub.3 magnetic powder, and more particularly to a convenient method for preparing a cheaper needle-shaped .gamma.-Fe.sub.2 O.sub.3 magnetic powder with a Ba-ferrite coating.
Magnetic recording is widely used in the computer-related products, such as magnetic tapes, and audio/video recording tapes. In prior audio applications, the .gamma.-Fe.sub.2 O.sub.3 having a coercivity of 200-400 Oe is generally used. In prior video applications, the Co-.gamma.-Fe.sub.2 O.sub.3 having a coercivity of 600-800 Oe is generally used. However, the Co-.gamma.-Fe.sub.2 O.sub.3 is unsuitable for high density recording because its coercivity cannot reach as high as 1500 Oe as required for high density recording purposes. For the demand of the new generation of particulate high density recording materials, such as in video and data storages, the magnetic powder having higher coercivity and smaller particle size needs to be studied and developed. Ba-ferrite magnetic powder is a candidate taken into consideration for those purposes (with respect to which reference is made to M. P. Sharrock, MRS Bulletin, Vol. XV, March 1990, P. 53).
However, the above-mentioned particulate recording media have the following drawbacks:
(a) The temperature coefficient of coercivity of the .gamma.-Fe.sub.2 O.sub.3 with cobalt additive is very high. In A. Eiling's article, IEEE Trans. Magn., MAG-23, No. 1, P. 16(1987), it is disclosed that the temperature coefficient of coercivity of the .gamma.-Fe.sub.2 O.sub.3 (with Hc=650 Oe) with an outer cobalt coating is -0.46%/.degree. C. while the temperature coefficient of the .gamma.-Fe.sub.2 O.sub.3 (with Hc=700 Oe) with cobalt additive is -0.71%/.degree. C. In M. P. Sharrock's article, IEEE Trans. Magn., MAG-25, P. 4374 (1989), it is disclosed that the temperature coefficient of coercivity of the .gamma.-Fe.sub.2 O.sub.3 with cobalt additive is -1.00%/.degree. C. while the temperature coefficient of the .gamma.-Fe.sub.2 O.sub.3 with an outer cobalt coating is from -0.2 to -0.5%/.degree. C. PA1 (b) The metallic powders have a serious dispersion problem, and are easily suffered from oxidation and corrosion, resulting in instability of properties. PA1 (c) The coercivity of the Ba-ferrite magnetic powder is too high, usually 4000-5000 Oe. The lowering of the coercivity thereof by dopants also lowers the saturation and residual magnetization thereof. In addition, there also is a dispersion problem.
The present invention intends to alleviate the drawbacks of the high temperature coefficient (from -0.2 to -1.0%/.degree. C.) and low coercivity (under 1000 Oe) of the conventional needle-shaped .gamma.-Fe.sub.2 O.sub.3 with cobalt additive (or with an outer cobalt coating).